The cutting insert used in milling cutter tools consists generally of a prismatic body having a planar base from which extend side surfaces--relief flank surfaces--which are intersected by the upper surface--the cutting rake surface. The cutting insert has at least one cutting edge which is defined as an edge created by the intersection of the cutting rake surface and the relief flank.
The insert is so held in the tool that the cutting edge is located in the circular cutting path of the tool and is inclined with reference to the rotary axis by an angle referred to as the axial rake angle.
The magnitude of the axial rake angle has a direct influence on the stability of the tool during operation and also on the durability of the cutting edge. In general, it is advantageous to have the inserts disposed at as large an axial rake angle as is compatible with avoiding structural weakening of the tool.
It is known that with inserts for milling cutter tools the relief angles employed are relatively large as compared with the angles used with stationary cutting tools, and this is particularly the case with milling tools having low diameter cutting paths. Furthermore, where the tool is to be used in milling highly ductile materials such as, for example, aluminium, even greater relief angles are required as compared to those indicated when, e.g., steel workpieces are to be milled. Increasing the relief angle, however, can lead to a weakening of the cutting edge and there is in consequence a limitation of the magnitude of the relief angle. On the other hand, a reduction in the magnitude of the relief angle tends to increase the wear surface of the insert in contact with the workpiece and this places a lower limit on the magnitude of the relief angle. In general, the magnitude of the relief angle is determined in accordance with the material of the workpiece, the material from which the cutting edge is formed and the cutting path diameter of the tool. Thus, with hard and tough workpiece materials, the relief angle is generally chosen to be between 12.degree. to 20.degree. with tools of high speed steel, and between 8.degree. to 12.degree. with carbide tools. On the other hand, with workpieces of mild steels, cast iron, etc., the relief angle can in fact be as high as 20.degree..
It has been found, however, that with peripheral rotary milling cutters, as a result of the location of the inserts so as to present an axial rake angle with respect to the longitudinal axis of the cutter, the relief angle presented by the cutting edge at its leading end is very much greater than the relief angle presented by the cutting edge at its opposite trailing end. On the assumption that the relatively smaller relief angle presented by the insert at its trailing end is, in fact, the minimum relief angle which should be employed, it will be realized that the relief angle at the leading end is unduly large, thereby leading to an increased danger of weakening of the cutting edge at this point and a consequent lowering of tool life.